What’s new in RSTP

Overview
Rapid spacing-tree protocol is an industrial standard originally define in IEEE 802.1W.  The same as STP, RSTP is a protocol to prevent loop happen at layer 2.  So we can build redundant loop free topology without worry about bridging loop.

What new in RSTP?
The 802.1D Spanning Tree Protocol (STP) convergent is so slow. When the link down, STP take time nearly a minute to recovery the connectivity. It’s so slow for the current network environment. Cisco enhanced the original 802.1D specification with features such as Uplink Fast, Backbone Fast, and Port Fast to speed up the convergence time of a bridged network. The drawback is that these mechanisms are proprietary and need additional configuration. With the new evolution of STP, RSTP simplified the network topology and make the convergence time faster.

If you already know about STP, you can rapidly configure the new protocol comfortably. It’s because of most parameter in RSTP have been left unchanged. Rapid Spanning-Tree protocol 802.1W can also revert back to 802.1D in order to interoperate with legacy bridges on a per-port basis.

The change in RSTP VS STP

  • Simplified port states
  • Additional port roles
  • Rapid convergence based on synchronisation process
  • Path calculation remains the same

Port State

STP RSTP
 Disable Discarding
 Blocking
 Listening Learning
 Learning
 Forwarding Forwording

RSTP define new port role
Beside Root Port & Designad port, RSTP define new port role as below:

  • Alternate
  • Backup
  • Edge

New RSTP Sync Process
Not like before root bridge send BPDU to other bridge to maintain topology, In RSTP all bridge sends proposals out all designated ports(synchronisation request).

  • In RSTP, each bridge generates BPDU every hello interval

     

     

    • 2 seconds by default
  • If 3 hellos are missed from a neighbour, reconvergence begins

     

     

    • 6 seconds vs 20 seconds MaxAge
  • MaxAge is used as hop count

     

     

    • Every bridge send BPDUs on its own
    • Age incremented by every bridge
    • MaxAge also used on shared ports for legacy STP Backwards compatibility
  • Faults can be detected faster by means of physical layer signaling

RSTP converengce

  • RSTP needs to re-converge when root port is lost
  • If there is an alternate port, it’s select in place of old root port

     

     

    • New root port is then synchronised with downstream bridges
  • If there are no Alternate ports and no better info

     

     

    • Declare it self as root
    • Synchronize this decision
    • Possibly adapt to better informatio

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